The rate of wear of the doctor blade in the various doctor units in a paper machine varies greatly. Depending on the doctor blade's position, its working life can vary from tens of hours to as much as tens of days. In terms of the operation of the paper machine and the doctor units, the degree of wear and general condition of the doctor blade would be valuable information. First of all, replacements could be predicted and, on the other hand, breakages could be noticed immediately. In the worst case, a worn-out or damaged doctor blade may be used, which will naturally lead to a poor doctoring result. At the same time, the doctor unit or even the surface being doctored can be damaged. If this results in the doctor dropping through, both the damage and the cost will be great. However, no effective method or device exists for determining the degree of wear of a doctor blade. Suggestions have been made for installing angle-sensors at the ends of the doctor unit. However, the change in the angle between the blade holder and the blade carrier does not indicate wear with sufficient accuracy. This is especially so, as the doctor blade usually wears least at the ends and generally most in the centre. In addition, the measurement of the angle will not show local faults or wear peaks in the doctor blade, which always lead to a poorer doctoring result. There is also no effective apparatus for monitoring the condition of the blade holder while the paper machine is running.
The wear of the blade and the doctoring result are particularly affected by the blade load and the blade angle being used at the time. Usually, the doctor blade is pressed against the surface being doctored by a load imposed on the blade holder by loading devices. In known doctor units, the loading devices are calibrated when the paper machine is stopped. The specific force, corresponding to the feed pressure of the loading devices, which is required to lift the doctor blade off the surface being doctored, is then generally measured. The results obtained can thus only be used to calculate the desired blade load approximately. The method can also be applied roughly to determine the blade load when running, but the method is complicated and inaccurate. The method also does not provide blade-load values over the width of the doctor unit, which would be important information for monitoring the doctoring result and the wear of the doctor blade.